1. Field of the Invention
This invention relates to a spool valve which has a simple structure and is easy to fabricate or machine and in which a transient flow of working oil can be smoothly controlled during a switching operation of a valve.
2. Description of the Prior Art
In general, a known spool valve defines, on a peripheral surface on its shaft, a land portion for sealing working oil and a passage portion for allowing working oil to pass between a valve bore and itself. In this type of spool valve, the flow of a working oil is regulated by opening or closing an end of port open to the bore with the land portion. In order for the spool valve to permit a smooth transient flow of working oil, special attention has been paid to the design of the spool valve.
For example, Japanese Utility Model Laid-open Publication Nos. Sho 61-181102 and Sho 61-191903 disclose a hydraulic control valve in which a land portion of a spool defines, at the end of its peripheral surface, a plurality of triangular notches. By causing the area of a flow passage of hydrualic oil to vary due to the notches, the flow rate of the hydraulic oil increased to decreased during the opening or closing operation of a port, thereby permitting a smooth flow of working oil.
However, in the conventional valve, the machining of the notches is complicated and difficult. In addition, it is difficult to dimension the notch which substantially determines the area of the passage, thus resulting in high costs.